Multiple frequency filter



0a. 25, 1949., G. R. CLARK 2,485,567

MULTIPLE FREQUENCY FILTER Filed Feb. 20, 1946 Eli ERG Y SOURCE IN V ENTOR. G/ABf/PT 1?. CZA/F/f ,6 [:LUn

A 7' TORNEY Patented Oct. 25, 1949 2,485,567 MULTIPLE FREQUENCY FILTERGilbert R. Clark, Nutley,

Telephone and Radio Corporation,

N. J., assignor to Federal New York,

N. Y., a corporation of Delaware Application February 20, 1946, SerialNo. 649,064

11 Claims.

This invention relates to electrical wave filter systems and moreparticularly to filter systems capable of providing relatively sharpfiltering for a plurality of low frequencies with separate outputs.

In my copending application, Serial Number 664,483, filed April 24,1946, I have disclosed a filtering system which is based on theaccumulative effect of stored oscillating energy which is delivered tothe system at a given frequency and at a constant phase. The filter asembodied is of the rotary type in its operation and is basicallysynchronous in that the storage of oscillating energy though in staticcondition takes place in synchronism with the frequency to be selectedand that the switching rate through the rotary operation is related tothat frequency. In the filter system of the above-identifiedapplication, I have disclosed a system which is adapted for theselection of a single frequency by providing a plurality of like storagemeans with which there is associated means for applying and taking offcyclically and in a predetermined order energy at a given rate withrespect to these storage means. The number of such storage means is suchas to make them responsive to a wave of a single frequency only, thespeed of the cycle of energy application and take-off being thedeterminate of the respective selected frequency.

The effect of this filter is to operate as a synchronous rectifier forrespective portions of the selected wave which are then combined into aresulting alternating current type wave. The wave in that instance isdivided into preferably three equal portions of 120. However, in certaincases,

the need arises for the effective selection of more than one frequency.

It is an object of the present invention to provide a filter systemwhich is highly selective in respect to a plurality of frequencies inthe audio band.

It is a further object of the invention to provide an electro-mechanicalrotary filter for the selection of a number of given frequencies.

It is a still further object to provide a rotary wave filter of the typedefined which provides a separate alternating current output for each ofa number of selected frequencies.

In accordance with certain features of the invention provide in aunitary assembly a plurality of storage means corresponding in number tothe number of frequencies it is desired to select. Each of the storagemeans, which is comprised of submultiple units is associated with energydistributors the number of which is equal to the number of frequenciesit is desired to select. Each of the distributors in turn, includes adistributing section and a series of segments whose number is divisibleexactly by the number of storage means submultiple units. A number ofenergy distributing arms is provided corresponding to the number offrequencies to be selected to distribute the incoming energy to thestorage means over the distributing elements. The arms are rotated froma common shaft at a speed which is related to the number of segments andthe frequencies to be selected. Take-off brushes are provided on thedistributing arms to provide take-off energy for suitable outputcircuits. The effect will be similar to the system referred to above inthat a synchronous rectification takes place for successive waveportions of a plurality of frequencies, separate square-shaped outputsbeing obtained which may be filtered back into a substantiallysinusoidal form quite readily.

While my invention is defined in the appended claims, and other featuresand objects of the invention the foregoing will become more apparentupon consideration of the following detailed description to be read inconnection with the accompanying drawings in which:

Fig. 1 represents in diagrammatic form an electro-mechanical filtersystem in accordance with one embodiment of the invention; and

Fig. 2 illustrates the input and output wave forms of the system of Fig.1.

Referring to Fig. 1, a source of oscillating electrical energy having inits output wave forms of various frequencies is indicated at l which maybe connected to input terminals 2 and 3 of a rotary multiple frequencyfilter 4 in accordance with the invention. As indicated for frequenciesfz and is, energy is applied over respective input resistances 5 and 6,connected, in parallel in respect to the input terminal 2, todistributing sectors l and 8. Similarly, although not shown, energy isapplied to the remaining four distributing sectors 9, If], H and I2which are arranged in a circle about a driving shaft l3. The energyapplied to the sectors 1 through [2 is cyclically stored on a number ofcondensers such as the three condensers l4, l5 and I5 associated withthe sector I, and on the condensers ll, 18 and I9, associated with thesector 8. Each of the six sectors has a series of segments 20 and 2|associated therewith whose number is exactly divisible by the number ofcondensers provided for that sector. Three condensers are shown here forillustrative purposes. In this case the number of segments should bedivisible exactly --into three portions of 120 by three. However, othernumbers of condensers may be used instead, in which case the segmentsshould be divisible by the particular numbers respectively; also eachfrequency section may use any number of condensers without regard to thenumber of condensers used for the other sections. The number ofdistributing segments for any one Of the sectors is determined inaccordance with the ratio of the various frequencies which are to beselected as will be explained at a later point. In order to transferenergy from the sectors to the storage condensers, six distributor arms22 are provided at equally spaced points about the shaft 13'. Arms 22are each provided with an input distributing brush '24 which transfersenergy from the sectors to the distributing segments as from 8 t 2|, andwith a take-off brush 25 which reads or takes-off energy from thesegments 20, 2| and transfers it to take-off or output sectors 26. Thetake-off sectors 25 are each connected to a separate output circuit 27as illustrated for the frequency is. The output circuit includes in eachcase a high value grounded resistance 28, the voltage available acrossthe resistance being applied to a control grid 28 of a triode amplifier30. The output from the triode may be obtained over a couplingtransformer as at 3|. Since the filter illustrated comprises threecondensers for storing energy for each of the frequencies, thedistributor segments 2%, 2i are interconnected in such a manner as toprovide contact thru the brushes to the same condenser every 120electrical degrees, the interconnections between the segments beingindicated at 32.

In Fig. 2, graph a, a sine wave is shown divided each corresponding tothe three condensers which have been provided in the filter of Fig. 1for each of the frequency waves that are to be selected. Each of thethree condensers acts as a synchronous rectifier for the respectivecomponent of the wave, to which it is periodically subjected through thebrush system as by means of the input distributing brush 24. Thesegments 2U, 21 which are interconnected so as to form three successivegroups separated by 120 electrical degrees are contacted by the inputbrushes at a cyclic repetition rate which is gove'ned by the speed ofrotation of the brush system. Since, as illustrated in Fig. 1 thesegments for each frequency differ in width that is, in arcuate extentaccording to the ratio of the various frequencies to one another, asingle speed being used for all the six brushes, the duration of contactis a different one for each of the frequencies. The condensers for eachof the frequencies therefore will be charged in synchronism with thefrequency to be selected.

When, for instance, the proper frequency, i2, is present at the inputterminals 2, 3, the rate of charge of the condensers l4, l5 and I6 viathe bridging brush 24 is governed by the resistor 5. The time ofbuild-up of stored energy is directly proportional to the value ofresistor 5, hence the band-width of the filter around frequency f2 isinversely proportional to the value of resistor 5.

Since a low impedance signal source is always connected across the inputterminals whether f2 is present or not, when f2 ceases to exist theenergy stored in condensers l4, l5 and l 6 proceeds to discharge (viathe bridging brush 24) through resistor 5 and the low impedance signalsource to ground. The decay of the filtered signal, thus, is at the samerate as the earlier build-up (likewise governed by resistor 5).

Since the output resistance 28 in the take-off circuit, is relativelyhigh, the condensers will not be appreciably discharged, the effectrather being that of a reading of the respective condenser voltageswhich is transmitted to the utilization circuit. Thus the wantedcomponent frequency will continue to build up charge in the condenserswhile unwanted components will not cumulatively build up therein.

The resulting take-off wave form as obtained in the coupling transformeris shown in graph b of Fig. 2, if an in-phase relation is maintainedbetween the rotating brush system and the desired frequency.

Ordinarily, when only the frequency of the resulting alternations isdesired, rather than a specific wave form, the resulting rectangularshape of the wave form as suggested in graphs b of Fig. 2 issatisfactory. This wave form, however, may be easily filtered back. intoa sinusoidal form if desired.

While the above is a description of the principles of this invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationon the scope of this invention.

I claim:

1. A rotary wave filter for the selection of a plurality of differentfrequencies from a source of oscillatory energy including at least saidplurality of frequencies, comprising for each frequency of saidplurality that is to be selected energy storage means having separatesubmultiple portions, means for applying energy cyclically from a givensource to each of said storage portions in synchronism with therecurring portions of the waves of the respective frequencies to beselected, and means for successively obtaining signals from each of saidstorage portions for each of said selected frequencies, said signalsbeing proportional to the energy stored in said energy storage portionsand in synchronism with the respective selected frequencies.

2. A rotary wave filter for the selection of a given plurality offrequencies from a source of oscillatory energy including at least saidplurality of frequencies, comprising energy distributing means havingcomponent portions equal in number to that of the frequencies to beselected, separate energy storage means associated respectively withsaid distributing means having separate submultiple portions, energytake-off means having component portions equal in number to that of thefrequencies to be selected, and an output circuit for each of theselected frequencies associated with said take-01f means portions.

3. A filter according to claim 2 in which the number of frequencies tobe selected is six and the number of said component portions of saiddistributing and said take-off means comprises six.

4. A filter according to claim 2 in which the number of saidsubmultiples of said storage means for any one of said given frequenciesis independent of the number used for the other frequencies.

5. A filter according to claim 2 in which the number of saidsubmultiples of said storage means comprises three.

6. A filter according to claim 2 in which said distributing meansincludes for each frequency a series of conductive segments of a numberwhich is exactly divisible by the number of submultiple portions of saidenergy storage means,

said segments being conductively interconnected to form cyclicallyrecurring groups effectively spaced a number of electrical degrees apartwhich is equal to the fraction of the wave of the particular frequencythe denominator of which is given by the number of submultiple storagemeans portions, the width of said segments being determined by the ratioof said frequencies.

7. A filter according to claim 2 in which said distributing meansincludes for each frequency to be selected an energy input distributingportion, a series of conductive segments of a number which is exactlydivisible by the number of submultiple portions of said energy storagemeans connected thereto, and conductive means for bridging said portionsand said segments arranged for movement with respect thereto at a speedrelated to the frequency to be selected.

8. A filter according to claim 2 in which said energy take-off meansincludes for each frequency to be selected an energy take-01f portion, aseries of conductive segments conductively associated with said storagemeans, and conductive means for bridging said portions and said segmentsarranged for movement with respect thereto at a speed related to thefrequency to be selected.

9. A filter according to claim 2 in which the component portions of saiddistributing means, the component portions of said take-off means arearranged in concentric circles, and said distributing and take-01f meanseach include a rotary contact element for each of said plurality offrequencies.

10. A rotary wave filter for the selection of six given frequencies froma source of oscillatory energy including at least said six frequencies,comprising for each of said six frequencies to be selected: an inputcircuit including a series resistor, an energy distributing portionconnected to said input circuit, a series of contact segments the numberof which is divisible exactly by three and which are interconnected soas to form cyclical groups spaced apart electrically for the respectivefrequencies, energy storage means comprising three condensers each ofwhich is connected to one of said contact segment groups, an energytake-off portion, an output circuit including a relatively highresistance connected to said take-off portion, and an electrical contactbrush each for providing a bridge between said distributing portion andsaid contact segments and a bridge between said segments and saidtake-oil portion respectively; said distributing portion, said take-offportion and said segments being arranged in concentric circlesrespectively; and said brushes for each frequency being mounted on asupporting arm for rotation in respect to all of said portions andsegments at a speed which is synchronous with the respectivefrequencies.

11. A rotary Wave filter for the selection of N given frequencies from asource of oscillatory energy including at least said N frequencies,comprising for each of said frequencies to be selected: an inputcircuit, a series of contact segments the number of which is divisibleexactly by N/ 2 and which are interconnected so as to form cyclicalgroups spaced apart electrically for the respective frequencies, energystorage means comprising N/2 condensers each of which is connected toone of said contact segment groups, and an electrical bridge betweensaid input circuit and said contact segments.

GILBERT R. CLARK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,851,092 Fetter Mar. 29, 19322,430,038 Wertz Nov. 4, 1947

